The present invention relates to a receiver having an interface arrangement for connection between said receiver and a conditional access sub-system.
In the evolution of the specifications for MAC-packet television transmission systems which are currently represented by the European Broadcasting Union documents "Television Standards for the Broadcasting Satellite Service--Specification of the C-MAC/Packet System"--SPB 284, 3rd revised version published December 1984 and "Methods of Conveying C-MAC/Packet Signals in Small and Large Community Antenna and Cable Network Installations"--SPB 352 published December 1984, which documents are incorporated herein by way of reference, the requirement to scramble vision, sound and data services for conditional access purposes was identified as an important requirement at an early stage. For this reason, all members of the MAC-packet family permit, using a standardized method of conveyance, the transmission of two types of entitlement message. These are:
(i) Entitlement Checking Messages (ECMs), which allow the conditional access sub-system to derive the control words for a service or set of services. These control words are 60 bits long, and are used as inputs to the descrambling circuitry contained within the decoder for the MAC signal. These are sent to all customers.
(ii) Entitlement Management Messages (EMMs), which can be addressed to specific customers or groups of customers. The use of the packet multiplex to convey these messages is generally referred to as "over-air addressing", although this is not the only conveyance method. For example, postal or telephony networks could be used--different broadcasters will possibly use different methods depending upon their operational preferences. Specific users and groups of users are identified respectively by a Unique Customer Address (UCA) of 36 bits and a Shared Customer Address (SCA) of 24 bits.
Each of these message types is allocated a different address within the packet multiplex. Within an r.f. channel there will usually be only one EMM service (and hence one EMM packet address) but in the most general case there can be several ECM channels, carried on different packet addresses, each one corresponding to a separately scrambled service set. It can readily be seen, therefore, that depending upon the operational conditions of individual broadcasters, the contents (in terms of data capacity) of the ECM and EMM channels may vary considerably. For example, where a broadcaster does not use over-air addressing EMM packets are not transmitted.